Tuliposides (Pos) are major defensive secondary metabolites in tulip (genus Tulipa ), having 4-hydroxy-2-methylenebutanoyl and/or (3 S )-3,4-dihydroxy-2-methylenebutanoyl groups at the C-1 and/or C-6 positions of d -glucose. The acyl group at the C-6 position is converted to antimicrobial lactones, tulipalins, by tuliposide-converting enzymes (TCEs). In the course of a survey of tulip tissue extracts to identify novel Pos, we found a minute high-performance liquid chromatography peak that disappeared following the action of a TCE, and whose retention time differed from those of known Pos. Spectroscopic analyses of the purified compound, as well as its enzymatic degradation products, revealed its structure as 5″- O -(6- O -(4′-hydroxy-2′-methylenebutanoyl))-β- d -glucopyranosyl-(2″ R )-2″-hydroxymethyl-4″-butyrolactone, which is a novel glucoside ester-type Pos. We gave this compound the trivial name ‘tuliposide G’ (PosG). PosG accumulated in bulbs, at markedly lower levels than 6-PosA (the major Pos in bulbs), but was not found in any other tissues. Quantification of PosG in bulbs of 52 types of tulip, including 30 cultivars ( Tulipa gesneriana ) and 22 wild Tulipa spp., resulted in the detection of PosG in 28 cultivars, while PosG was present only in three wild species belonging to the subgenus Tulipa , the same subgenus to which tulip cultivars belong, suggesting the potential usefulness of PosG as a chemotaxonomic marker in tulip.